The word polycythemia simply means "many cells in the blood." There are two forms of this disease: polycythemia vera and secondary polycythemia.
Polycythemia vera (also called primary polycythemia) is a rare growth disorder of the bone marrow, occurring when the marrow is overactive and produces more blood cells than the body needs.
Secondary polycythemia affects some people as a result of limited oxygen due to smoking or living at high altitudes.
Polycythemia vera usually produces a high concentration of red blood cells or hemoglobin in the circulating blood, but it's important to note that white blood cell and platelet counts may also be increased.